Activation of the multicomponent antigen receptor on T cells (TCR) results in rapid activation of polyphosphoinositide metabolism and stimulation of a protein tyrosine kinase. We have characterized cells bearing abnormal antigen receptors lacking one or more receptor subunits and can relate defects in phosphoinositol release to absence of the TCR eta chain. Analysis of the tyrosine kinase pathway in T cells has revealed that the subunit of the antigen receptor phosphorylated on tyrosine residues after activation is the TCR zeta chain. In addition to this zeta chain phosphorylation, activation of the TCR results in rapid tyrosine phosphorylation of a 62 kD cytosolic protein which is currently being isolated. In addition to these studies high level expression of constructs containing the v-src kinase have been prepared and tyrosine phosphatases have been characterized. These multiple approaches have been undertaken in order to fully understand signal transduction and cellular activation in T cells.